The electric power steering apparatus is a machine which operates the motor for generating the steering assist force according to a steering torque applied by a driver. The recent years have seen a fast-growing demand for electric power steering apparatuses for use in large cars. Such a large car requires an increased steering assist force so that a larger amount of electric power must be supplied to the motor. In some cases, however, a battery alone may be unable to fully meet such a large power demand. In this connection, an arrangement has been proposed wherein an auxiliary power supply is provided in addition to the battery such that the normal power demand is met by the battery alone but when a larger amount of electric power is required, the electric power is supplied by both the battery and the auxiliary power supply connected in series (see, for example, Japanese Unexamined Patent Publication No. 2005-287222 (FIG. 1)).
When the voltage of the auxiliary power supply is lowered in the mode wherein the electric power is supplied by the battery and auxiliary power supply connected in series, the hitherto known electric power steering apparatus, as described above, provides control to maintain a predetermined electric power by increasing the current. Accordingly, an excess current may be extracted from the battery, which may be shortened in service life. In some cases, fuse for circuit protection may also blow.